Circadian rhythms are daily oscillations in behavior with a nearly 24-hour period that are generated by an internal biological clock. Across many organisms, health and fitness are impaired when the circadian clock does not appropriately synchronize with the daily cycles in the external environment. However, the mechanisms underlying these health costs are not clear. We are pursuing the hypothesis that a key function of the circadian clock is to optimize metabolism in anticipation of predicted changes in the environment. We are combining biophysical and biochemical measurements with experimentally-driven mathematically modeling in the bacterial model organism Synechococcus elongatus to study the interlocking relationships between clocks, metabolism, and organismal fitness. In this organism, the relationship between the clock and metabolism is very direct, and the core circadian oscillator can be reconstituted in a test tube using purified protein, making this an exceptionally power system to study this hypothesis. In Aim 1, we will determine the molecular mechanisms that maintain robust rhythms in the presence of metabolic signals in vitro using purified proteins. In Aim 2, we will determine the metabolic effects of clock mutants in vivo and their role in a metabolic feedback loop coupling clock output to input. In Aim 3, we will determine the impact of misalignment between the clock and metabolism on the reproductive rate and capacity of single cells in order to link these findings to organismal fitness.